1. Field of the Invention
The present invention relates to an accelerometer used for a control system.
2. Description of the Prior Art
An acceleration sensor is used for a control system such as an automotive air-bag system. The acceleration sensor has a sensing unit, an amplifier connected to the sensing unit, and a signal processing unit. The sensing unit is a semiconductor piezoresistance sensing unit formed by etching a semiconductor substrate into a stationary part, a thin cantilever beam extending from the stationary part (semiconductor post), a weight supported by the beam, and a piezoresistance bridge formed on the beam. Acceleration applied orthogonally to the surface of the sensor produces stress on the beam. The stress is detected by the piezoresistance bridge circuit and is amplified by the amplifier. The output of the amplifier is processed by the signal processing unit, which has an A/D converter and a signal processing circuit.
The air-bag system employs the acceleration sensor to measure impact deceleration at a collision. The sensing unit provides a signal representing the impact deceleration, which is amplified by the amplifier. According to the amplified signal, the signal processing unit decides whether or not to deploy the air-bag, and provides an instruction signal to a controller for the air-bag inflater.
The signal supplied to the signal processing unit must correctly represent the detected impact acceleration (deceleration). To achieve this, the sensing unit must have a specified sensitivity. The sensing unit, however, usually involves an error in sensitivity. The error is corrected by adjusting the amplification factor of the amplifier. When the amplifier includes a preamplifier and an operational amplifier, the error is corrected by adjusting the ratio of the resistance of an input resistor to that of a feedback resistor of the operational amplifier. For this purpose, the amplifier is provided with an extra output terminal at, for example, the intermediate point between the preamplifier and the input resistor of the operational amplifier. The extra output terminal is used to measure the sensitivity of the sensing unit.
The prior art corrects the sensitivity of the sensing unit in two steps. "A sensitivity measuring step" employs a vibrator to apply specific acceleration to the sensing unit, and a sensitivity detector to measure the sensitivity of the sensing unit according to a signal from the extra output terminal of the amplifier, and measures sensitivity data by the sensitivity detector. "A sensitivity adjusting step" calculates an amplification factor according to the measured sensitivity data, selects an input resistor and a feedback resistor for the operational amplifier based on the calculated amplification factor, and installs the selected resistors in the amplifier. This acceleration sensor still involves various errors such as an error in the sensitivity detector and errors in the input and feedback resistors, and therefore, is not accurate.
Another prior art installs a memory in the signal processing unit of the acceleration sensor. The sensitivity of the sensing unit is measured and stored in the memory. In operation, the signal processing unit reads the sensitivity out of the memory and corrects the output of the sensing unit accordingly. To store the sensitivity data in the memory, this prior art must employ a manipulation with a vibrator to apply specific acceleration to the sensing unit, as well as a sensitivity detector to measure the sensitivity of the sensing unit according to a signal from the extra output terminal of the amplifier. This acceleration sensor, therefore, involves an error in the sensitivity detector.